Solid-state lighting devices such as light-emitting diodes (LEDs) are increasingly used in both consumer and commercial applications. Advancements in LED technology have resulted in highly efficient and mechanically robust light sources with a long service life. Accordingly, modern LEDs have enabled a variety of new display applications and are being increasingly utilized for general illumination applications, often replacing incandescent and fluorescent light sources.
LEDs are solid-state devices that convert electrical energy to light and generally include one or more active layers of semiconductor material (or an active region) arranged between oppositely doped n-type and p-type layers. When a bias is applied across the doped layers, holes and electrons are injected into the one or more active layers where they recombine to generate emissions such as visible light or ultraviolet emissions. An LED chip typically includes an active region that may be fabricated, for example, from silicon carbide, gallium nitride, gallium phosphide, aluminum nitride, gallium arsenide-based materials, and/or from organic semiconductor materials. Photons generated by the active region are initiated in all directions.
Typically, it is desirable to operate LEDs at the highest light emission efficiency possible, which can be measured by the emission intensity in relation to the output power (e.g., in lumens per watt). A practical goal to enhance emission efficiency is to maximize extraction of light emitted by the active region in the direction of the desired transmission of light. Light extraction and external quantum efficiency of an LED can be limited by a number of factors, including internal reflection. According to the well-understood implications of Snell's law, photons reaching the surface (interface) between an LED surface and the surrounding environment are either refracted or internally reflected. If photons are internally reflected in a repeated manner, then such photons eventually are absorbed and never provide visible light that exits an LED.
One way to increase light extraction efficiency is to provide reflective surfaces that reflect generated light so that such light may contribute to useful emission in a desired direction from an LED chip. In a typical LED package 10 illustrated in FIG. 1, a single LED chip 12 is mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wire bonds 11 can connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The reflective cup 13 may be filled with an encapsulant material 16, which may contain a wavelength conversion material such as a phosphor. At least some light emitted by the LED chip 12 at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly is then encapsulated in a clear protective resin 14, which may be molded in the shape of a lens to collimate the light emitted from the LED chip 12. While the reflective cup 13 may direct light in an upward direction, optical losses may occur when the light is reflected. Some light may be absorbed by the reflective cup 13 due to the less than 100% reflectivity of practical reflector surfaces. Some metals can have less than 95% reflectivity in the wavelength range of interest.
FIG. 2 shows another LED package 20 in which one or more LED chips 22 can be mounted onto a carrier such as a printed circuit board (PCB) carrier, substrate, or submount 23. A metal reflector 24 mounted on the submount 23 surrounds the LED chips 22 and reflects light emitted by the LED chips 22 away from the LED package 20. The reflector 24 also provides mechanical protection to the LED chips 22. One or more wire bond connections 11 are made between ohmic contacts on the LED chips 22 and electrical traces 25A, 25B on the submount 23. The mounted LED chips 22 are then covered with an encapsulant 26, which may provide environmental and mechanical protection to the LED chips 22 while also acting as a lens. The metal reflector 24 is typically attached to the carrier by means of a solder or epoxy bond. The metal reflector 24 may also experience optical losses when the light is reflected because it also has less than 100% reflectivity.
FIG. 3 shows another LED package 30 in which an LED 32 can be mounted on a submount 34 with a hemispheric lens 36 formed over it. The LED 32 can be coated by a conversion material that can convert all or most of the light from the LED 32. The hemispheric lens 36 is arranged to reduce total internal reflection of light. The lens 36 is made relatively large compared to the LED 32 so that the LED 32 approximates a point light source under the lens 36. As a result, an increased amount of LED light that reaches the surface of the lens 36 emits from the lens 36 on a first pass. Additionally, the lens 36 can be useful for directing light emission from the LED 32 in a desired emission pattern for the LED package 30.
The art continues to seek improved LEDs and solid-state lighting devices having reduced optical losses and providing desirable illumination characteristics capable of overcoming challenges associated with conventional lighting devices.